Apparatus and method for producing ii-vi crystals

ABSTRACT

APPARATUS IS DISCLOSED FOR NUCLEATING AND GROWING II-VI CRYSTALS SUCH AS ZINC SELENIDE. THE APPARATUS COMPRISES A HOLLOW REACTION VESSEL HAVING WALLS CONSTRUCTED OF AN INERT, HIGH-TEMPERATURE RESISTANT MATERIAL AND HAVING AN OUTLET PASSAGEWAY WITH A MUCH SMALLER CROSSSECTIONAL AREA THAN THAT OF THE REACTION VESSEL, THE REDUTION IN AREA OCCURING IMMEDIATELY ADJACENT TO THE POINT OF CRYSTAL NUCLEATION. THIS APPARATUS ALSO CONTAINS A MEANS FOR HEATING THE REACTION VESSEL TO TEMPERATURES OF AT LEAST ABOUT 400*C. A PROCESS FOR NUCLEATING AND GROWING II-VI CRYSTALS USING A THROTTLING TECHNIQUE IS ALSO DISCLOSED. THE APPARATUS AND PROCESS DISCLOSED ARE USEFUL FOR PRODUCING SINGLE, PURE, DEFECT-FREE II-VI CRYSTALS WHICH ARE IN TURN USEFUL IN ELECTRO-OPTIC APPLICATIONS.

P; VOHL 3,677,717

APPARATUS AND METHOD FOR PRODUCING II-VI CRYSTALS July 18, 1972 FiledMarch 6, 1969 mmtkmI Q 4 QR mi t0 bavsQkw PA UL VOHL INVENTOR.

ATTOR/VE X 3,677,717 APPARATUS AND METHOD FOR PRODUCING II-VI CRYSTALSPaul Vol11, Lexington, Mass., assignor to Itelr Corporation, Lexington,Mass. Filed Mar. 6, 1969, Ser. No. 804,895 Int. Cl. BOlj 17/30 U.S. Cl.23-294 13 Claims ABSTRACT OF THE DISCLOSURE Apparatus is disclosed fornucleating and growing II-VI crystals such as zinc selenide. Theapparatus comprises a hollow reaction vessel having walls constructed ofan inert, high-temperature resistant material and having an outletpassageway with a much smaller crosssectional area than that of thereaction vessel, the reduction in area occurring immediately adjacent tothe point of crystal nucleation. This apparatus also contains a meansfor heating the reaction vessel to temperatures of at least about 400 C.

A process for nucleating and growing IIVI crystals using a throttlingtechnique is also disclosed.

The apparatus and process disclosed are useful for producing single,pure, defect-free IIVI crystals which are in turn useful inelectro-optic applications.

BACKGROUND OF THE INVENTION (I) Field of the invention This inventionrelates to an apparatus and process for producing II-VI crystals andmore particularly relates to an apparatus and process for producingsingle, pure, defeet-free II-VI crystals by a vapor transport technique.

(II) Description of the prior art Nucleation and growth of II-VIcrystals has heretofore been accomplished by the use of temperaturegradients. For example, Piper and Polich have disclosed an apparatuswherein a sealed crucible containing a II-VI source material and anucleation point is moved through a temperature gradient. SeeVapor-Phase Growth of Single Crystals of II-VI Compounds, Journal ofApplied Physics, vol. 32, No. 7, July 1961, pp. 1278-9. Nucleation andgrowth with this apparatus and technique is directly proportional to therate at which the crucible is moved through the temperature gradient.

De Meis and Fischer have taught an apparatus for preparing II-VIcrystals which consists of a quartz ampoule having a sealed capillarytube attached to one end of the ampoule. See Improved Method for GrowingII-VI Crystals From the Vapor Phase, Materials Research Bulletin, vol.2, 1967, pp. 465-8. The purpose of the capillary in this apparatus is toadjust the minimum vapor pressure within the system by collecting excessvolatile material until the decomposition pressure in the ampoule hasreached its minimum. In use, the ampoule is evacuated to a pressure of10" Torr, sealed, and moved through a temperature gradient furnace.Crystal growth initiates in the hotter portion of the capillary tubethereby sealing it off.

Frosch, U.S. 3,197,411, issued July 27, 1965, teaches an apparatus forgrowing single crystals of the III-V serni conductor compounds, galliumphosphide and gallium arsenide, on suitable substrates. This apparatusconsists of a fused silica tube which has gas inlets and which extendsthrough two temperature controlled furnaces designed to produce atemperature gradient within the apparatus. The source is contained in aquartz boat at a high temperature end. The substrate material ispositioned further down the quartz tube at a lower temperature. A

United States Patent 3,677,717 Patented July 18, 1972 small gas outletis positioned well beyond the substrate where the crystal nucleates andgrows. Nucleation and growth of the III-V crystals is caused by thetemperature gradient maintained between the source and growth surface(see col. 2, lines 31-33). Since the apparatus operates on a thermalgradient principle, it can function equally well as both an open systemand closed system (see col. 2, lines 28-31).

All of the above-described apparatus and processes depend on saturatedvapor in a temperature gradient for crystal nucleation and growth. Nonesuggest that crystallization from the vapor phase can be accomplished byother techniques.

SUMMARY OF THE INVENTION It has now been found that nucleation andgrowth of single II-VI crystals from a carrier gas saturated withvaporized source material can be accomplished by simply throttling thecarrier and vapor species. This is completely unexpected.

Relying on this finding, a novel and unique apparatus for nucleating andgrowing crystals containing at least one Group II-B element and at leastone Group VI-A element selected from sulphur, selenium, and telluriumhas been invented.

This apparatus comprises a hollow reaction vessel having wallsconstructed from an inert, high-temperature resistant material such asquartz. A carrier gas is admitted to the reaction vessel through glasstubing or by other means and is exited from the reaction vessel througha passageway having a much smaller cross-sectional area (A than that ofthe reaction vessel (A The reduction in areas creates a substantialthrottling effect on the carrier gas and other vapor species which inturn causes crystal nucleation and growth to commence. Heating means areused to vaporize the source material.

A process for nucleating and growing II-VI crystals from a sourcematerial containing a Group II-B element or a Group VI-A elementselected from sulphur, selenium, and tellurium has also been invented.This process takes advantage of the throttling phenomenon.

In the process, the source is contacted with a flow of carrier gas andheated to a temperature sufiicient to saturate the vapor phase with theelements to be crystallized. The saturated vapor phase is then throttledto cause crystal nucleation and growth.

The apparatus and process of this invention olfer many advantages overwhat has been heretofore known in the prior art. For example, highergrowth rates with concomitant shortened running times can be obtainedwith the claimed apparatus and process. Another important advantage isthat crystal grows freei.e. mechanically unconstrained by chamber wallsor crucibles and such free growth results in improved crystalproperties. Furthermore, the apparatus is simple in construction andoperation and therefore offers a more conomical means of growing crystals. Another advantage is that the apparatus can be maintained atatmospheric pressure thereby eliminating complicated and expensiveequipment to maintain low vacuums or superatmospheric pressures. Afurther advantage of this apparatus and process is that the singlecrystals of II-VI materials produced are unusually pure and defectfree.Also very large crystals can be grown. Additionally, the apparatus andprocess herein disclosed make seeding unnecessary to obtain crystalnucleation. Also, the system automatically insures that stoichiometricconditions are maintained because excess volatile materials areexhausted. Exhausting the excess volatile materials also results inhigher purity crystals. While the excess volatile materials areexhausted, it has been found that very little of the vaporized sourcematerial escapes from the apparatus of this invention. This is becausethe vaporized source material is cooled in the exit means and thereforecondenses before it can escape to the open atmosphere.

A further advantage to the apparatus and process of this invention isthat they provide a means to achieve the nucleation and growth of asingle crystal nucleus at a desired, predetermined point location.Furthermore, they prevent extraneous nucleation at other undesirablepoints.

BRIEF DESCRIPTION OF THE FIGURE The figure is a partially cut awayschematic diagram of apparatus for nucleating and growing lI-VI crystalsaccording to this invention.

DESCRIPTION OF THE INVENTION This invention is related to an apparatusand a process for growing crystals of II-VI elements. For purposes ofthis invention, the elements referred to as II or Group II elements aremore specifically those elements of Group II-B-i.e., zinc, cadmium andmercury. In a like manner, the VI or Group VI elements useful with thisinvention are more specifically Group VI-A elements selected fromsulfur, selenium and tellurium. The terms II, VI, Group II-B and GroupVI-A refer to groupings found in a periodic chart of the elements suchas the one found opposite page 1 of The Condensed Chemical Dictionary,7th ed. For brevity, the crystals produced will often be referred to asII-VI crystals.

The apparatus claimed contains a hollow reaction vessel having wallsconstructed from an inert high temperature resistant material such asglass, quartz, or a ceramic material such as mullite. This vessel has across-sectional area A Of course, the vessel does not have to have acircular cross-sectional area, although this is one preferredembodiment, Square, triangular, elliptical, etc. cross-section areaswould also be suitable for the reaction vessel, as would any other shapeincluding irregular shapes.

A means for admitting a carrier gas is connected to the front portion ofthe apparatus. Any suitable means such as glass, plastic, or rubbertubing are suitable to allow entrance of the carrier gas. A particularlypreferred means for admitting the carrier gas is a capillary tubeintegrally attached to the front of the reaction vessel because such acapillary helps to prevent back difiusion of the carrier gas andvaporized material due to temperature gradients within the reactionvessel.

An exit passageway is provided to allow the carrier gas to leave thereaction vessel. This exit passageway has a crosssectional area A whichis sufficiently smaller than A to create a throttling effect on thecarrier gas and vaporized source material as they pass from the reactionvessel into the exit passageway. It is this throttling effect thatcauses nucleation to commence. The theory of why such a throttlingefiect causes nucleation is not presently known, but it has beenrepeatedly observed that nucleation commences immediately adjacent tothe point of such throttling. Because of this, it is important toposition the reduction of cross-sectional areas from the reaction vesselto the exit means immediately adjacent to the point at which it isdesired to nucleate and grow the crystal.

The cross-sectional areas of the reaction vessel and exit means do nothave to be uniform. Where they are not however, the areas A and A arethe areas immediately prior to and immediately past the location wherethe reaction vessel and exit means are joined. The reduction in areasfrom A to A has to be substantial to attain crystal nucleation andgrowth. It has been found that a ratio of A /A should normally be in therange of from about 1/40 to about 1/ 10,000, and preferably is in therange of from about 1/ 3,000 to about 1/5,000 for the best results.

The apparatus also has a means of heating the reaction vessel totemperatures of at least about 400 C. and to much higher temperatures ifthey are required to vaporize specific source materials. The heatingmeans can be any well-known type of heater such as a resistance heater,inductance heater, etc. A particularly preferred heating means is onewhich can provide a temperature gradient within the reaction vessel. Thehigher temperature is maintained at the source and the lower temperatureis maintained at the nucleation'and growth end of the reaction vessel.Although a temperature gradient is not required, it has been found toresult in faster crystal growth rates, especially after the initialnucleation and growth period.

The proper temperatures for the source and point of crystallization willdepend, of course, upon many factors including the particular Group lI-Band Group VI-A elements to be crystallized. In general, the source willhave to be heated to and maintained at a temperature suflicient toachieve a vapor phase saturated with a Group II-B element and a GroupVI-A element and the point of crystallization can be maintained at aboutthe same temperature or preferably at a somewhat lower temperature. Asone illustration of this, it has been found desirable to maintain a zincselenide source in the range of from about 1150 C. to about 1230 C.while maintaining the point of crystallization at a temperature of about0-50 C. lower.

One embodiment of the apparatus for producing single II-VI crystals isshown in the figure. The reaction vessel 1 comprises a cylindrical pieceof quartz tubing with a capillary tube 2 integrally attached to it toallow the gas to enter the reaction vessel and a capillary tube 3 alsointegrally attached to it to allow the carrier gas to pass from thereaction vessel. The source material 4 is positioned at the front end ofthe apparatus and can be contained or uncontained. A heater 5 ispositioned about the reaction vessel. As the carrier gas enters thereaction vessel 1 through the capillary tube 2, and passes the heatedsource 4, it transports vaporized source material from the heated sourcematerial 4. The carrier gas transports the vaporized source materialthrough the reaction vessel to the joining point for the capillary tube3 and reaction vessel 1, where the carrier gas and vaporized sourcematerial are throttled causing nucleation to commence at point 7. Thesingle crystal 8 grows inwardly towards the center of the reactionvessel 1 unconstrained by the vessel walls. An inlet tube 9 and anoutlet tube 10 can be respectively positioned before the capillary tube2 and after the capillary tube 3. Such inlet and outlet tubes simplyserve to admit and exhaust the carrier gas.

It has been found that apparatus as described above with entrance andexit capillaries having inside diameters of about 0.25 to about 3millimeters and with a reaction vessel having an inside diameter ofabout 5 to about millimeters, all being circular in cross-section,produce excellent results.

A process for nucleating and growing lI V I crystals from a sourcematerial as described above will now be described.

Initially, the source material is heated to a temperature suflicient tosaturate the vapor phase while the source is contacted with a fiow of aninert carrier gas. The vaporized source material is transported by thecarrier gas, and nucleation and crystal growth are initiated bythrottling the flow of carrier and vaporized source material.

For this process, the source must contain either a Group II-B element orone of the Group VI-A elements. It may contain more than one element ofeach group. If it only contains one of the Group [LB or Group Vl-Aelements, the other required element to product the II-VI crystals canbe introduced as part of the carrier gas. In the preferred embodiment ofthis invention, a Group II-B and a Group VI-A element are included inthe source material.

As stated above, the source material is heated to an elevatedtemperature. The exact temperature will vary with the particular sourcematerial, but will be known or easily ascertainable by those skilled inthe art.

The carrier gas flow is introduced so that it comes into intimatecontact with the heated source material. Since the source material hasbeen heated to an elevated temperature the vapor species becomesaturated with vaporized source material.

Many gases are sufiicient as carriers including hydrogen, nitrogen,argon, and helium. Others will be apparent to those skilled in the art.

The carrier gas can contain a Group II-B element or a Group VI-A elementwhich is to form part of the crystal being grown. Hydrogen sulfide (H 8)and hydrogen selenide (H Se) are examples of such gases. Additionally,

doping gases or gases containing a constituent useful in forming mixedcrystals can be used as carrier gases.

The saturated vapor phase is transported by the carrier gas from thesource to a crystallization point. As explained above, the temperatureat the point of crystallization does not have to differ from that at thesource to obtain nucleation. Preferably, however, the temperature at thepoint of crystallization is somewhat lower than that at the sourcebecause such a temperature gradient results in faster crystal growth.

At the point where crystallization is desired, the carrier gas andsaturated vapor phase are throttled sufficiently to cause nucleation.This can be accomplished by any suitable means, as for example, byforcing the gas and source vapor through an orifice or capillary tube.Nucleation commences immediately adjacent to the point at whichthrottling occurs.

Although the process and apparatus of applicant have heretofore beendescribed in terms of producing II-VI crystals containing at least oneGroup II-B element and one Group VI-A element, it will be clear to thoseskilled in the art that mixed crystals can also be produced. Someexamples of such mixed crystals include those having the followingformulas:

where x and y each have a numerical value greater than zero and lessthan one.

It will also be clear to those skilled in the art that the II-VIcrystals can be doped with various other elements. This can beaccomplished by a variety of techniques which would include, forexample, placing a doping material inside the reaction vessel close toor with the source material or by introducing a doping material combinedwith the carrier gas. Some examples of well-known doping agents includein particular the elements of boron, aluminum, gallium, indium,phosphorus, arsenic, antimony, and copper as well as many others knownto those skilled in the art.

The apparatus and process described herein are both useful in producingsingle, pure II-VI crystals of exceptionally large size. These crystalsare in turn useful in electro-optic applications such asimage-converters, image storage and retrieval systems, andlight-emitting devices. The following example illustrates the invention.All parts and percentages are by weight unless otherwise specified.

EXAMPLE The reaction vessel is a quartz ampoule cylindrical in shape andhaving a uniform inside diameter of about 30 millimeters along itslength of about 9 centimeters. Capillary tubes 0.5 millimeter indiameter and about 3 centimeters in length are integrally attached atboth ends. The reaction ampoule also has a side arm attachment for thepurpose of loading the source material. Six millimeter diameter quartzend tubes are attached to the capillary tubes leading from the reactionampoule. The reaction vessel is surrounded with a wire wound resistanceheated furnace about 14 inches in length with a core opening of 2inches. The furnace is suitably insulated with fire brick. The quartzend tube protrudes out of the furnace to provide a room temperaturecondensation zone for residual vapors.

Approximately 40 grams of powdered or sintered zinc selenide is loadedinto the reaction ampoule through the side arm. The side arm is thensealed off near the ampoule and removed. All traces of source powder arecarefully removed from the deposition zone by torching while a reverseflow of argon drives the vapor to the front end of the reaction chamber.The ampoule is then installed into the furnace and hydrogen gas linesare attached to the end tubes. The furnace is raised to a temperature ofabout 1220 C. Hydrogen fiow is maintained at about 3 cubic centimetersper minute.

Nucleation is observed to have occurred at the entrance to the exitcapillary tube after two days. The crystal continues to grow towards thecenter of the reaction ampoule. While some crystallites do condense inthe capillary tube, at no time during the run is the capillary tubeclogged. Vapor flow is continuous throughout the run. As the crystalgrows into the space of the reaction ampoule, the throttling elfectdiminishes and growth can be accelerated by cooling the growth area to atemperature of 0 to 50 lower than the temperature at the sourcematerial. The run is continued under these conditions for a total of 8days at which time a single, pure defect-free zinc selenide crystal of20 grams is produced.

What is claimed is:

1. Apparatus for using a vapor transport technique to nucleate and growcrystals of at least one Group II-B element and at least one Group Vl-Aelement selected from sulfur, selenium, and tellurium, said apparatuscomprising:

(a) a hollow reaction vessel having walls constructed from an inert,high-temperature resistant material, said reaction vessel having across-sectional area A (b) means for admitting a carrier gas to saidreaction vessel;

(c) an exit passageway having a single flow area with an effectivediameter of from about 0.25 millimeter to about 3 millimeters and havinga cross-sectional area A which is sufiiciently smaller than A to createa substantial throttling effect on the carrier gas and vaporized sourcematerial, the ratio A /A being in the range of from about 19, to about 6the reduction from A to A being located immediately adjacent to thepoint at which crystal nucleation and growth is desired;

(d) means for heating said reaction vessel to temperatures of at leastabout 400 C., said means for heating being capable of producing atemperature gradient within the reaction vessel, the temperature at thesource being higher than the temperature at the point of crystalnucleation and growth.

2. Apparatus of claim 1 wherein said means for admittlng a carrier gasis a capillary tube having an insde diameter of from about 0.25millimeter to about 3 millimeters.

3. Apparatus of claim 2 wherein said reaction vessel has an insidediameter at A of from about 5 millimeters to about millimeters.

4. Apparatus of claim 3 wherein said mens for heating the reactionvessel comprises a resistance heater.

5. A process for nucleating and growing II-VI crystals from a vaporphase saturated with at least one Group II-B element and at least oneGroup VI-A element selected from sulfur, selenium and tellurium,comprising:

(a) contacting a source of said Group II-B and said Group VI-A elementswith a flow of carrier gas;

(b) heating said source to a temperature sufficient to achieve a vaporphase saturated with said Group II-B and said VI-A elements;

(c) reducing the fiow area at the point of condensation by providing asingle fiow area at said point having an effective diameter of fromabout 0.25 millimeter to about 3 millimeters to throttle the fiow ofsaid saturated vapor phase and to limit nucleation thereby effectingsingle crystal growth.

6. A process of claim wherein the temperature at the source ismaintained higher than the temperature at the point of crystalnucleation and growth.

7. A process of claim 6 wherein said source material contains zincselenide.

8. A process of claim 6 wherein said source material contains zincsulfide.

9. A process of claim 6 wherein said source material contains zinctelluride.

10. A process of claim 7 wherein said carrier gas is selected from thegroup consisting of hydrogen, nitrogen, helium and argon.

11. A process of claim 7 wherein said carrier gas c-omprises hydrogen.

12. A process of claim 11 wherein said source is maintained at atemperature in the range of from about 1 150 C. to about 1230 C.

13. A process of claim 12 wherein said point of crystal nucleation andgrowth is maintained at a temperature of from 0 C. to about C. lowerthan the temperature maintained at the source material.

References Cited UNITED STATES PATENTS 3,031,261 4/1962 Vogel et a123-294 3,058,812 10/1962 Li Chu et al. 23--273 3,104,365 9/ 1963 Broseret a1. 23-294 3,199,961 8/ 1965 MacAvoy et al. 23 294 3,218,204 11/1965Ruehrwcin 23294 3,224,912 12/1965 Ruehrwein 23-294 3,338,761 8/ 1967Cheney et al. 23273 3,353,914 11/1967 Pickar 23-601 3,362,795 H1968Weisbeck 23--301 3,525,594 8/1970 Barrett 23-273 NORMAN YUDKOFF, PrimaryExaminer R. T. POSTER, Assistant Examiner U.S. Cl. X.R. 23273, 301, 305

